Rollup merge of rust-lang#51753 - gruberb:document-from-conversions-libstdpath, r=QuietMisdreavus

Document `From` implementations

This PR is solves part of rust-lang#51430. It's my first PR, so I might need some guidance from @skade (as already mentioned in the issue).

The purpose of the PR is to document the `impl From` inside `path.rs` and answering the questions:
- What does it convert?
- Does it allocate memory?
- How expensive are the allocations?

I gave it a first shot, though an experienced rust developer might want to look over it.

Author: pietroalbini

src/libstd/path.rs

@@ -1397,13 +1397,20 @@ impl<'a> From<&'a Path> for Box<Path> {
 
 #[stable(feature = "path_buf_from_box", since = "1.18.0")]
 impl From<Box<Path>> for PathBuf {
+    /// Converts a `Box<Path>` into a `PathBuf`
+    ///
+    /// This conversion does not allocate or copy memory.
     fn from(boxed: Box<Path>) -> PathBuf {
         boxed.into_path_buf()
     }
 }
 
 #[stable(feature = "box_from_path_buf", since = "1.20.0")]
 impl From<PathBuf> for Box<Path> {
+    /// Converts a `PathBuf` into a `Box<Path>`
+    ///
+    /// This conversion currently should not allocate memory,
+    /// but this behavior is not guaranteed on all platforms or in all future versions.
     fn from(p: PathBuf) -> Box<Path> {
         p.into_boxed_path()
     }
@@ -1426,20 +1433,29 @@ impl<'a, T: ?Sized + AsRef<OsStr>> From<&'a T> for PathBuf {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl From<OsString> for PathBuf {
+    /// Converts a `OsString` into a `PathBuf`
+    ///
+    /// This conversion does not allocate or copy memory.
     fn from(s: OsString) -> PathBuf {
         PathBuf { inner: s }
     }
 }
 
 #[stable(feature = "from_path_buf_for_os_string", since = "1.14.0")]
 impl From<PathBuf> for OsString {
+    /// Converts a `PathBuf` into a `OsString`
+    ///
+    /// This conversion does not allocate or copy memory.
     fn from(path_buf : PathBuf) -> OsString {
         path_buf.inner
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl From<String> for PathBuf {
+    /// Converts a `String` into a `PathBuf`
+    ///
+    /// This conversion does not allocate or copy memory.
     fn from(s: String) -> PathBuf {
         PathBuf::from(OsString::from(s))
     }
@@ -1536,6 +1552,7 @@ impl<'a> From<Cow<'a, Path>> for PathBuf {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<PathBuf> for Arc<Path> {
+    /// Converts a Path into a Rc by copying the Path data into a new Rc buffer.
     #[inline]
     fn from(s: PathBuf) -> Arc<Path> {
         let arc: Arc<OsStr> = Arc::from(s.into_os_string());
@@ -1545,6 +1562,7 @@ impl From<PathBuf> for Arc<Path> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl<'a> From<&'a Path> for Arc<Path> {
+    /// Converts a Path into a Rc by copying the Path data into a new Rc buffer.
     #[inline]
     fn from(s: &Path) -> Arc<Path> {
         let arc: Arc<OsStr> = Arc::from(s.as_os_str());
@@ -1554,6 +1572,7 @@ impl<'a> From<&'a Path> for Arc<Path> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<PathBuf> for Rc<Path> {
+    /// Converts a Path into a Rc by copying the Path data into a new Rc buffer.
     #[inline]
     fn from(s: PathBuf) -> Rc<Path> {
         let rc: Rc<OsStr> = Rc::from(s.into_os_string());
@@ -1563,6 +1582,7 @@ impl From<PathBuf> for Rc<Path> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl<'a> From<&'a Path> for Rc<Path> {
+    /// Converts a Path into a Rc by copying the Path data into a new Rc buffer.
     #[inline]
     fn from(s: &Path) -> Rc<Path> {
         let rc: Rc<OsStr> = Rc::from(s.as_os_str());